1. Field of the Invention
This invention relates to a method and system for conducting event-streamed spectrum imaging (“ESSI”). More particularly, this invention relates to a method and system for conducting such imaging using concurrent collection of electron and spectral signals.
2. Discussion of the Prior Art
Spectrum imaging is the collection and spatial registration of all spectral events, yielding a spectral data cube. By the term “spectral events” is meant the converted value of the measurement of a physical property of a photon or elemental particle. The physical property could be the energy of an x-ray photon, the energy of an electron, the wavelength of a light photon, the mass of an ionized atom as well as other similar physical properties. For the use in this discussion, spectral events are the converted x-ray energies from an x-ray detector/pulse processor, the value of which is proportional to the energy of the x-ray. Various analytical methods can be applied to the spectral data cube, ranging from simple elemental region-of-interest images, to spectral summation of the pixel elemental weight percent, to true chemical phase images. By the term “region-of-interest images” is meant a region defined with regard to a span of x-ray energies that corresponds to peak location of an element in an x-ray spectrum. The sum of x-ray counts over the defined energy region is collected for each pixel, creating an element image. By the term “spectral summation of the pixel elemental weight percent” is meant the summation of the x-ray spectra that correspond to pixels inside a spatially defined region of interest. The resultant x-ray spectrum can then be quantified to yield the weight percent values of the constituent elemental distribution. By the term “true chemical phase images” is meant the processing of the spectral data cube by methods such as principal components or multi-variant statistical analysis, both of which use statistical methods to transform the data into a basis where it can be visualized according to an eigenvector formulation.
Collecting a spectrum image involves scan generation which is the process of generating an x-y spatial raster scan over an area of interest using such methods of electron/ion beam scanning or specimen stage scanning. Typically, scan generation is used to collect an image from any signal where the source of the signal is converted to analog from either a backscattered or secondary electron detector, but it can be a signal from any detector (e.g., absorbed current, EBIC, or cathodoluminescence detectors) connected to the microscope. With traditional spectrum imaging, the signal source is the converted x-ray energies. The time required to collect a spectrum image is dependent on the x-ray photon flux, the amount of x-ray dwell time per pixel, the image size and the number of image frames scanned.
There are two methods in the prior art known for collecting a spectrum image. In the first method, the spectrum image is collected sequentially (Ingram et al., Microbeam Analysis, 1988, Hunt and Williams, Ultramicroscopy, 1991), that is, pixel by pixel. At each pixel position, a full spectrum of spectral events is collected for a given x-ray dwell time. Sequential spectrum imaging has a disadvantage of requiring long collection times per pixel in collecting the entire image mostly due to significant overhead in transferring a full spectrum of spectral events for each pixel.
In the second method, the spectrum image is collected using the method of position-tagged spectrometry (Legge and Hammond, Journal of Microscopy, 1979, Mott et al., Proceedings Microscopy and Microanalysis, 1995) in which spectral events are tagged with the corresponding pixel position in spatial x, y coordinates while the pixels are continually scanned. Pixels that contain no spectral events are not collected. By the term “tagged” is meant the pixel positions are passed to the spectral signal processor that performs the operation of associating the pixel position with the spectral event when the spectral event occurs.
The disadvantage of both methods is that they lack integration with scan generation and electron imaging. These methods solely focus on the collection of spectral data without attaching importance to the simultaneous inclusion of other signals of interest, such as secondary and backscattered electron signals, for a given pixel position. This does not allow viewing spectral and electron information in tandem on a display, limiting the ability to exactly associate any features or artifacts found in the spectrum image with those occurring in the electron image. Separately acquiring spectrum and electron images may reduce the reliability of information that is inferred from each other about associated features or artifacts.
A further disadvantage is that the data collected by both methods is not efficiently organized for storage or subsequent processing for display and analysis at the host. To overcome these limitations and increase the speed, accuracy and relevance of information obtained from analyzing spectrum images the method of event-streamed spectrum imaging discussed and described herein has been developed.